Radar images of composite targets tend to be quite different from their visual counterparts. Since directional illumination is used and since target surfaces are fairly smooth in terms of wavelengths, only a small number of regions on the target reflect in the radar direction at any particular target orientation. The radar image appears to be composed of a finite number of point scatterers in contrast to the visual image, which is continuous over the object surface. In general radar targets are three dimensional, hence, the distribution of point scatterers is three dimensional.
Imaging is one method of target identification by radar. Of all available methods it carries the highest information content. Although a radar image comparable to the visual image is desirable, there are limits to achieving this. Due to the much lower target-size-to-wavelength ratio at radar wavelengths, and due to the more specular nature of radar echoes, a radar image will, at best, crudely approximate the visual image and generally require interpretation or comparison with known radar images of that target for identification. Radar images are typically, but not exclusively, two dimensional views of a target where the two dimensions are range (resolvable by radar bandwidth) and crossrange (resolvable by Doppler spectral shift). This assumes that the target rotates around an axis perpendicular to the radar beam direction. The radar image plane contains the radar beam direction. This is in contrast to a visual image, which is orthogonal to the viewing direction, assuming that the target produces a two dimensional silhouette-type image. In the invention described here we use a unique form of target motion to develop a two dimensional radar image in a plane orthogonal to the radar beam direction. This image is of the visual type. Its resolution depends on the radar frequency but not on its bandwidth.
A state-of-the-art account of "High Resolution Radar Imaging" can be found in a book of that title by Dean L. Mensa, published by Artech House Inc., Dedham, Mass. Described herein (with respect to FIG. 1) is a method disclosed by Mensa for range/crossrange imaging with a single-wavelength radar, which will illustrate some general principles and which is related to this invention. The Mensa method is published under the title "Aperture Synthesis by Object Rotation in Coherent Imaging", IEEE Transactions on Nuclear Science, Vol. NS-27, No. 2, pp 989-998, April 1980. Mensa teaches that a CW radar can generate two dimensional images of a captive target which rotates around an axis perpendicular to the radar beam direction.